The present invention relates to metrology tools with rotating stages having integrated z-axis adjustment for inspecting a wafer.
This invention relates to optical metrology tools of the type described in U.S. Pat. No. 6,278,519, incorporated herein by reference. Referring to prior art FIG. 1, these types of tools include a light source for generating a probe beam 7, which is focused onto a semiconductor wafer 4. Changes between the incident probe beam 7 and the reflected beam are monitored to evaluate characteristics of the sample 4.
Tools of this type typically include a motion stage for supporting the wafer 4. Various stage motion combinations are available including full X-Y stages; R/theta stages; and xc2xd X-{fraction (1/2)}Y  plus theta stages (where theta means 360 degrees of rotation). Prior art FIG. 1 exemplarily illustrates an X-stage 22, a Y-stage 24, and theta stage 26. The motion of the stages is computer controlled for moving the wafer into position with respect to the focused spot of the probe beam 7.
These tools also typically include a focusing (preferably autofocusing) system, which brings the wafer into the focal plane of the focusing optics of the measurement system 2. A number of these systems operate to translate the focusing optics in a vertical direction with respect to the sample. Alternatively, the stages themselves are provided with some form of vertical (z-axis) movement for focusing purposes. Since the motion system needs to be designed to fit within the available height 3, conventional Z-axis stages that utilize guide rails are difficult to integrate. The length of the profile moving along the guide rails directly affects the Z-axis"" stiffness against tilting movement. Where the length of the moving profile is limited by the available height, the tilting movement of the moving parts becomes hard to control. In order to reduce the tilting movement, the contact pressure between the moving profile and the guide rails needs to be increased, which results in increased friction and consequently increased actuating forces. High friction and actuating forces again reduce the movement resolution in Z-axis.
Therefore, there exists a need for an apparatus and method for highly precise vertical micro adjustment of a rotating stage with minimal friction and a maximum stiffness against tilt movement and lateral movement.
Conventional linear guiding systems define the movement direction by either a sliding or a rolling contact. This is feasible where an extensive movement range needs to be covered. In this application, the required Z-axis movement range is only twenty thousandths of an inch. Providing Z-axis movement over that small a range with sliding or rolling guides still requires a relatively bulky and heavy assembly, which increases the moment of inertia of the motion system. As a consequence, the motion system moves more slowly.
Therefore, there exists a need for a Z-axis guiding system that is low in mass as well.
A wafer motion system includes one or two conventional linear stages and a rotating stage, which are mounted on top of each other. The one or two linear axes are horizontal. The rotating stage is placed at the top and is configured for holding a wafer and rotating it around a vertical axis of revolution. The one or two linear stages have a travel range defined in combination with the rotating stage to position the wafer with respect to the probe beam. The wafer is placed on a chuck and held down by a vacuum provided between wafer and chuck.
The chuck itself is guided along the axis of revolution within the rotating stage. Specifically configured and placed flexures or membranes elastically guide the chuck without any substantial friction. The membranes easily deflect in the vertical direction while being highly rigid in horizontal direction. Preferably, at least two ring shaped membranes are vertically positioned relative to each other. The horizontal stiffness of each membrane in combination with the vertical offset between them results in a high stiffness against tilt.
The rotationally symmetric design of the membranes allows them to be easily integrated into the generally rotationally symmetric design of the rotating stage. The relatively small mounting space required for mounting the membranes results in little additional volume and mass necessary for integrating the membrane rings in the rotating portion of the rotating stage.
The membranes provide a substantially friction free guidance of the chuck allowing for a smooth and precise actuation and adjustment. A horizontally oriented piezo stack is utilized to provide the vertical actuation of the chuck via a lever system, which amplifies and transforms the horizontal expansion of the piezo stack into a vertical movement of the required range. The vertical lever movement is transmitted to the chuck unit via a central linking assembly, which provides for initial adjustment and preload of the actuator to the chuck. The linking assembly also receives external vacuum and/or pressure air and transmits it into the chuck unit.
The vertical movement system is actuated by a voltage applied to the piezo stack, which expands in accordance to the well-known principles of piezo elements. The amount of horizontal piezo stack movement is in the micron range. The lever system amplifies the piezo movement by a factor of approximately 15.